Seat heaters are utilized in mobile and stationary seating, furniture, and beds. Many seat heaters include a generally straight heating wire that is attached to a woven or non-woven fabric carrier or other pliable substrate material. In this class of heaters, the heating wire is retained on the textile surface by thread sewn back-and-forth over the wire such that the wire position and the wire sewing line (“route”) is predetermined by a computer-controlled sewing apparatus. This technology allows sewing of the initially straight wire along a straight or curved route. The wire is fed onto the carrier along the predetermined route and held in the desired pattern by the sewing thread as it is sewn. However, in practice the minimum radius of the wire sewing line is about 5 mm and normally it is even larger, for example within 10-15 mm.
In one known heater design, the application of a wire that is preformed with miniature waves and a bend radius below 5 mm has been found advantageous. Such a wire shape allows for “packing” more wire length into the seat heater, which reduces the maximum wire temperature, allows for the use of cheaper alloys, and makes the seat heater more durable, especially if a seat in which the heater is installed is prone to create a crease in the heater. This type of heating wire may be referred to as a “formed,” “preformed,” or “processed” wire. In this heater design, the formed heating wire is laminated onto a substrate or in-between two layers of substrate by an adhesive or thermally activated adhesive.
In tests performed on a formed corrugated copper alloy wire with a 7 mm wave “magnitude,” a 1 mm bending radius, and an 8 mm bending period, the formed wire lasted 2-10 times longer than the straight versions of the same wires. The straight wire is less durable and upon bending, it breaks, whereas a corrugated wire when bent tends to twist, making it more durable. However, so far no success has been reported in attaching such a corrugated wire by a sewing technology.
At least three reasons prevent conventional sewing technology from achieving a wire route replicating such advantageous tiny waves and sharp turns that formed wire provides. The first reason is that the wire stiffness and the tendency of the wire to retain its straightness makes the wire deviate from the desired route to the extent of being hit by the sewing needle. The second reason is that sharper curves require smaller sewing zig-zag pitch (a step between two subsequent hits by the needle), which slows down the sewing process. The third reason is that the wire metal strands may be deformed within their elasticity range and thus the wire continues over time to attempt to unbend and return to a straight disposition. This may cause a shift of the wire or the moving of the seat heater out of its position in a seat years after its installation.